With advances in user interface technologies and computer-based medical diagnostic methods, there is an increasing demand for accurate, fast, portable and inexpensive eye trackers and fixation monitors. Because eye gaze is a strong indication for current attention and intention, such devices may automatically and accurately estimate: where the person is looking, the current and past areas of attention, the possible intentions and/or the possible neurological stability of a person. Eye tracking thus provides a key input to enable a range of applications and devices that would benefit from utilizing such information. The scope of potential applications is extensive, ranging from medical diagnostics to intuitive and fast man-machine interfacing. Examples include mobile devices, computer interaction in professional environments, security applications, vehicle security and vehicle interaction, air traffic control, computer gaming, etc. Presently, eye tracking already provides great value in commercial and research-related applications such as psychology and vision research, assistive technology, eye-based communication for people with highly limited mobility, commercial usability, and advertising studies, etc.
Gaze direction can be estimated by a variety of techniques, each of them having its advantages and limitations. Most contemporary eye trackers detect eye position, usually employing the reflection of a point light source from the front surface of the cornea (corneal light reflex) relative to the bright or dark pupil (when the eye rotates, the pupil moves about twice as fast as the corneal light reflex, with the differential being a function of the direction and amount of eye movement), or relative to the reflection of the same point light source from the back of the crystalline lens of the eye (fourth Purkinje image).
More precise are the foveal eye trackers. When an individual looks at a target, that target is imaged on the fovea. It is thus foveal fixation that correlates precisely with gaze direction. It has also been shown that landmarks such as the fovea and the optic disc can be detected robustly by measuring the amount of polarization change caused by the surrounding birefringent nerve fibers during double passage of a beam of light through them upon fundus reflection in double-pass systems. Recent research has shown that techniques that effectively track or monitor the optical projection of fundus landmarks out from the eye afford a more direct measurement of fixation direction, and are physiologically more relevant. The major advantage of this new eye-fixation detection and tracking method is that it uses true information coming directly from retinal landmarks, as opposed to existing eye-tracking systems that use reflections from other structures, to identify the direction of foveal gaze.
Current non-invasive video eye trackers use digital, image-based sensors and can be relatively fast and accurate. Among them are the EyeLink 1000 Plus of SR Research (2 kHz max, after a costly upgrade from 1000 Hz), EYE-TRAC 7 of Applied Scientific Laboratories (360 Hz max), TX300 of Tobii (300 Hz max), 3D ETD of Chronos Vision GmbH (400 Hz), Hi-Speed 500 from SensoMotoric Instruments (500 Hz), and others. Yet, they are laboratory instruments that cost tens of thousands of dollars and are either cumbersome tabletop units or delicate, head-mounted devices, unsuitable for use in many patients, especially in children. For many applications in ophthalmology, neurology, otology, and neuro-otology, measurement speeds of several thousand measurements per second are highly desirable, often for an extended period of time, i.e. a minute or more. Example are studying saccades, post-saccadic oscillations, fixation stability with age-related macular degeneration, pursuit eye movement, etc. Acquiring complete digital images at a high frame rate inevitably puts a restriction on the recording time and the throughput of the system. Today, such speed without high bandwidth streaming video can only be provided by the more invasive scleral search coil recordings, which require the subject to sit within a metal antenna frame while a coil of wire is placed on the eye for measurement under exacting conditions. Scleral search coils induce discomfort and impact the eye movement and the ability to maintain convergence. Children cannot tolerate scleral search coils.
All existing eye-tracking instruments are designed to determine and track the direction of gaze of one or both eyes, requiring cooperation by the subject for precise calibration. They usually record the accuracy of fixation on a directed target.
A method and device are therefore needed for fast and accurate eye tracking and fixation monitoring, without requiring digital streaming, storage and manipulation of complete images at high frame rates, but rather acquisition and transmission of only sufficient data needed for X-Y tracking of the pupil.